The present invention relates generally to magnetic resonance imaging (MRI) systems and, more specifically, to a design for a radio frequency coil for an open magnet MRI system.
Magnetic resonance imaging systems provide images of internal structures of the human body and the like by detecting radio signals produced by the precessing spins of the atomic nuclei of the structure when the structure is placed in a strong polarizing magnetic field. The nuclear spins are first excited into precession by a radio frequency (RF) stimulation pulse. Next the spins are isolated spatially by application of one or more gradient magnetic fields that cause their precession frequency to deviate from that provided by the polarizing magnetic field alone. The isolated resonance signals produced by these precessing nuclear spins are detected and processed according to techniques well known in the art to produce tomographic or volumetric images.
A single antenna may be used to transmit the stimulating RF pulse and to receive the weaker resonance signals from the precessing nuclei although often separate antennas are used for these two purposes.
In a prior art xe2x80x9cclosedxe2x80x9d MRI system, a polarized gradient magnetic field is produced by a cylindrical, annular magnet having a bore for admitting a patient along the axis of the cylinder aligned with the magnetic field B0. Nuclei precession within the patient is induced by an RF field providing a magnetic vector in a plane perpendicular to the B0 axis.
For certain procedures, particularly surgical procedures, an xe2x80x9copenxe2x80x9d MRI system may be desired in which the annular magnet of the closed MRI system is replaced by opposed magnetic pole faces providing therebetween a relatively unobstructed opening into which a patient may be placed while preserving greater access to the patient than in a closed MRI system. In the open MRI system, the B0 field extends between the pole faces and the RF field is kept perpendicular to the B0 field.
In open MRI systems, to avoid unduly restricting access to the patient through the opening between opposed magnetic pole faces, one or more arrays of parallel conductors positioned near the pole faces are used to provide the RF field. These conductors are energized in a manner that produces a net RF vector in the desired plane perpendicular to the B0 axis.
While a single opposed pair of RF coils may be used for producing an oscillating RF field along a single line, preferably each such RF coil is matched to a second array having perpendicularly running conductor elements. For the RF stimulating pulse, the two matched RF coils are energized with signals having a 90 degree phase difference so as to create a rotating RF field. For reception of the resonance signal, signals detected at the crossing RF coils are combined with the appropriate 90 degree phase difference to produce a signal with superior signal-to-noise ratio. Coils providing for perpendicular reception or transmission patterns are known generally as xe2x80x9cquadraturexe2x80x9d coils.
A radio frequency shield may be placed between the RF coils and coils that produce the gradient magnetic field described above, so as to prevent signal from the gradient coils from interfering with reception of signals by the RF coils. Such radio frequency shields may be used as a return conductor path for an RF coil.
While open frame MRI systems provide greater access to a patient for surgical and other procedures than closed MRI systems, providing a high degree of homogeneity for the radio frequency and magnetic fields necessary for high quality imaging is still a challenge. In this regard, it is important that the pole faces be as close as possible to each other, and therefore that the RF coils and radio frequency shield be as close as possible to each other as well. Providing this homogenous RF reception and transmission field with a compact coil structure remains an important area of development.
A number of improvements to the design of quadrature coils suitable for open frame MRI systems are set forth herein.
While it is not possible to produce the ideally desired perfectly uniform RF field between the pole faces, conductor patterns designed to approximate the geometry of uniform current sheets parallel to the magnet pole faces are herein used to achieve a high degree of approximation to the desired RF field over the central imaging region.
Although the conductor elements of each coil array of a quadrature coil will be perpendicular and therefore theoretically isolated, in fact there exists significant capacitive coupling between such elements, particularly when the elements are placed in close proximity as is desired in an open frame MRI system. A first feature of the invention is an isolation circuit canceling out this capacitive effect.
Conventional termination of the conductor elements of the arrays is unduly resistive and/or promotes unequal current flow through these elements, limiting homogeneity of the resulting field. Accordingly, a second feature of the invention is an improved termination for these conductor elements that provides greater and more equal current flow. Additionally, a series connection between the coil arrays ensures identically matching current flows through the upper and lower corresponding conductor elements. An effective RF shield is provided for such quadrature coils which accommodates both transmission of magnetic field gradients and reduction of interaction between the gradient coils and the RF coil.
Specifically, a quadrature RF coil for an open MRI system is provided. The MRI system includes a polarizing magnet with opposed pole faces for establishing a polarizing field axis. The coil includes a first conductor array having separate and substantially aligned conductor elements positioned along a first conductor axis and extending across the polarizing field axis between opposed common connection points. A second conductor array includes separated and substantially aligned conductor elements positioned along a second conductor axis extending across the polarizing field axis between opposed common connection points, and extending perpendicularly to the first conductor elements. A combiner/splitter electrically coupled to a connection point of each of the first and second conductor arrays joins them with a common signal line so that a signal path between the common signal line and the connection point of the first conductor array is substantially 90 degrees out of phase with a signal path between the common signal line and the connection point of the second conductor array.
An isolation circuit joins the connection points of each of the first and second conductor arrays to create between the first and second conductor arrays a blocking parallel resonance at the operating radio frequency. The isolation circuit may comprise an adjustable inductor for providing parallel resonance in combination with a parasitic capacitive coupling between the overlying conductors of the first and second conductor array. For flexibility in tuning this circuit, a fixed or variable capacitor may be added between the first and second conductor arrays so as to be coupled in parallel with the parasitic capacitance.
Thus the invention, in one embodiment, constitutes an extremely compact planar coil suitable for use in open MRI systems providing high signal-to-noise ratio and quadrature detection. Because an extremely low profile RF coil may be constructed if parasitic capacitance between the elements is overcome, insertion of the inductor to convert this parasitic capacitance into a blocking parallel resonant circuit at the RF frequency, effectively eliminates its effect at the frequencies of interest.
Ideally, the radio-frequency body coil would produce a perfectly uniform magnetic field with a direction perpendicular to the static magnetic field produced by the magnetic pole faces. The direction perpendicular to the pole faces is parallel to the static magnetic field and is taken as the direction of the z-axis in a Cartesian coordinate system. A uniform, infinite, y-directed sheet of current with surface current density xcexy does not produce any magnetic field in the y or z directions. The field in the x-direction is given by the expression
Bx=xcexcoxcexy for z greater than zo
and
Bx=xe2x88x92xcexcoxcexy for z less than zo.
Therefore, two such current sheets with equal but oppositely directed current densities, one located at z=zo, slightly below the upper pole face, and the other at z=xe2x88x92zo, slightly above the lower pole face, will produce a magnetic field
Bx=2xcexcoxcexy for xe2x88x92zo less than z less than zo
and
Bx=0 for z less than xe2x88x92zo or z greater than zo.
In theoretical terms this idealized pair of current sheets is an optimized source for the radio-frequency field of an open MR scanner from two points of view:
(1) The field between the current sheets is completely uniform and independent of position.
(2) The current sheets provide no obstruction to the region of the gap between the pole faces (xe2x88x92zo less than z less than zo). However, because of its infinite extent, a coil consisting of such a pair of current sheets is not a practical design for an MR scanner. Furthermore, a large area conducting sheet of metal such as copper would shield the imaging region from the fields of the switched gradient coils which are typically required in MR imaging and which are located in the space between the RF coils and the magnetic pole faces. In a preferred embodiment of the invention, practical coil designs are provided which approximate the desirable properties of the pair of infinite uniform current sheets as just described.
A pair of coils, each with its primary conducting elements located within a rectangular region near to and parallel with the magnet pole faces, can form a practical approximation to the ideal pair of current sheets. This region is taken to be of width W in the x-direction and length L in the y-direction. A number N of equally spaced conductor strips, each parallel to the y-axis and extending from y=xe2x88x92L/2 to y=L/2 and each carrying the same y-directed current, are placed within this rectangle and arranged symmetrically around, and parallel to, the y-axis. The same pattern, but with oppositely directed currents, is placed on the lower pole face. By increasing the number of strips so that the space between them becomes negligible and allowing W and L to become arbitrarily large, the magnetic field pattern of this coil pair approaches that of the ideal pair of conducting sheets discussed above. If N is odd, there will be a conducting strip on each coil at the x-location given by xo=0 and an additional (Nxe2x88x921)/2 pairs of conducting strips located at xo(n)=xc2x1n W/(Nxe2x88x921) for 1xe2x89xa6nxe2x89xa6(Nxe2x88x921)/2. If N is even there will be on each coil N/2 pairs of strips at xo(n)=xc2x1(nxe2x88x921/2) W/(Nxe2x88x921) for 1xe2x89xa6nxe2x89xa6N/2.
Because the parallel sets of conductors just described do not form closed electric circuits, it is necessary to provide additional conducting elements whose purpose is not primarily to produce the magnetic field in the imaging region but, rather, to close the conducting circuits of each of these two conductor arrays. A number of alternative possibilities are available for completing the circuit paths and the most desirable means of doing this will depend on the particular imaging application and system design being utilized. If the current elements closing the path are located remotely from the region of imaging, the field in the imaging region will be substantially that of the linear conductor arrays. This field is described below.
Applying the Biot-Savart law to a single linear current element which extends in the y-direction from y=xe2x88x92L/2 to y=L/2 and is located at x=xo and z=zo leads to the following expressions for the magnetic field components at the field point (x,y,z).                               B          x                =                  xe2x80x83                ⁢                                            μ              o                                      4              ⁢              π                                ⁢                      xe2x80x83                    ⁢                                    z              -                              z                o                                                                                      (                                      x                    -                                          x                      o                                                        )                                2                            +                                                (                                      z                    -                                          z                      o                                                        )                                2                                                                                      xe2x80x83                ⁢                  [                                                                      L                  /                  2                                -                y                                                                                                        (                                                                        L                          /                          2                                                -                        y                                            )                                        2                                    +                                                            (                                              x                        -                                                  x                          o                                                                    )                                        2                                    +                                                            (                                              z                        -                                                  z                          o                                                                    )                                        2                                                                        +                                                            L                  /                  2                                +                y                                                                                                        (                                                                        L                          /                          2                                                +                        y                                            )                                        2                                    +                                                            (                                              x                        -                                                  x                          o                                                                    )                                        2                                    +                                                            (                                              z                        -                                                  z                          o                                                                    )                                        2                                                                                ]                                                  B          y                =                  xe2x80x83                ⁢        0                                          B          z                =                  xe2x80x83                ⁢                                            μ              o                                      4              ⁢              π                                ⁢                      xe2x80x83                    ⁢                                    x              -                              x                o                                                                                      (                                      x                    -                                          x                      o                                                        )                                2                            +                                                (                                      z                    -                                          z                      o                                                        )                                2                                                                                      xe2x80x83                ⁢                              [                                                                                L                    /                    2                                    -                  y                                                                                                                    (                                                                              L                            /                            2                                                    -                          y                                                )                                            2                                        +                                                                  (                                                  x                          -                                                      x                            o                                                                          )                                            2                                        +                                                                  (                                                  z                          -                                                      z                            o                                                                          )                                            2                                                                                  +                                                                    L                    /                    2                                    +                  y                                                                                                                    (                                                                              L                            /                            2                                                    +                          y                                                )                                            2                                        +                                                                  (                                                  x                          -                                                      x                            o                                                                          )                                            2                                        +                                                                  (                                                  z                          -                                                      z                            o                                                                          )                                            2                                                                                            ]                    .                    
A complete coil pair will contain N linear conductors at xo=xo(n) and z=zo and N additional conductors at xo=xo(n) and z=xe2x88x92zo where n runs from n=1 to n=N. The total field produced by the two linear arrays is then given by             B      x        =                            ∑                      n            =            1                    N                ⁢                              B            x                    ⁢                      (                                                            x                  o                                ⁢                                  (                  n                  )                                            ,                              z                o                                      )                              +                        ∑                      n            =            1                    N                ⁢                              B            x                    ⁢                      (                                                            x                  o                                ⁢                                  (                  n                  )                                            ,                              -                                  z                  o                                                      )                                          B      y        =    0              B      z        =                            ∑                      n            =            1                    N                ⁢                              B            z                    ⁢                      (                                                            x                  o                                ⁢                                  (                  n                  )                                            ,                              z                o                                      )                              +                        ∑                      n            =            1                    N                ⁢                                            B              z                        ⁢                          (                                                                    x                    o                                    ⁢                                      (                    n                    )                                                  ,                                  -                                      z                    o                                                              )                                .                    
For a single conducting element, if the length L of the conductor becomes very long compared to the quantities (xxe2x88x92xo) and (zxe2x88x92zo), then             B      x        →                            μ          o                          2          ⁢          π                    ⁢              xe2x80x83            ⁢                        z          -                      z            o                                                              (                              x                -                                  x                  o                                            )                        2                    +                                    (                              z                -                                  z                  o                                            )                        2                                          B      y        =    0              B      z        →                            μ          o                          2          ⁢          π                    ⁢              xe2x80x83            ⁢                                    x            -                          x              o                                                                          (                                  x                  -                                      x                    o                                                  )                            2                        +                                          (                                  z                  -                                      z                    o                                                  )                            2                                      .            
At the center of the imaging volume (x,y,z)=(0,0,0) and the central field of an individual conducting strip is given by             B      x        =                  -                  xe2x80x83                ⁢                              μ            o                                4            ⁢            π                              ⁢              xe2x80x83            ⁢                        L          ⁢                      xe2x80x83                    ⁢                      z            o                                                (                                          x                o                2                            +                              z                o                2                                      )                    ⁢                                    (                                                                    L                    2                                    4                                +                                  x                  o                  2                                +                                  z                  o                  2                                            )                                      1              /              2                                                      B      y        =    0              B      z        =                            μ          o                          4          ⁢          π                    ⁢              xe2x80x83            ⁢                                    L            ⁢                          xe2x80x83                        ⁢                          x              o                                                          (                                                x                  o                  2                                +                                  z                  o                  2                                            )                        ⁢                                          (                                                                            L                      2                                        4                                    +                                      x                    o                    2                                    +                                      z                    o                    2                                                  )                                            1                /                2                                                    .            
If N is even in the symmetry of the inventive coil, all current elements can be grouped in groups of four wires with positive currents at (xo,zo) and (xe2x88x92xo,zo) and negative currents at (xo,xe2x88x92zo) and (xe2x88x92xo,xe2x88x92zo). This group of four wires produces a central field given by             B      x        =                  -                  xe2x80x83                ⁢                              μ            o                    π                    ⁢              xe2x80x83            ⁢                        L          ⁢                      xe2x80x83                    ⁢                      z            o                                                (                                          x                o                2                            +                              z                o                2                                      )                    ⁢                                    (                                                                    L                    2                                    4                                +                                  x                  o                  2                                +                                  z                  o                  2                                            )                                      1              /              2                                                      B      y        =    0              B      z        =    0.  
The total central field is determined by summing over all of the groups of four wires that are present in the coil. If N is odd, there is an additional contribution from the pair of wires at (xo=0, zo) and (xo=0,xe2x88x92zo) which must also be added to the field of the other conductors. The central field of this wire pair is             B      x        =                  -                  xe2x80x83                ⁢                              μ            o                                2            ⁢            π                              ⁢              xe2x80x83            ⁢                        L          ⁢                      xe2x80x83                    ⁢                      z            o                                                (                                          z                o                2                            ⁢                              (                                                                            L                      2                                        4                                    +                                      z                    o                    2                                                  )                                      )                                1            /            2                                          B      y        =    0              B      z        =    0.  
Therefore, this coil geometry, as desired, produces a magnetic field that is predominately in the x-direction near the center of the magnet gap.
The first and second conductor arrays may be comprised of copper foil laminated to opposite sides of a planar insulating substrate, as typified in conventional printed circuit technology, and the isolation circuit may be coupled to adjacent common connection points on opposite sides of the planar insulating substrate. This simplifies fabrication of extremely compact quadrature coils for open frame MRI systems. The printed circuit technology registers the first and second conductor arrays precisely with respect to each other and allows the isolation circuit to operate by connecting to adjacent coil ends through a small aperture in the insulating substrate.
The conductor elements of each conductor array may be connected together via a first node connection connecting the first ends of the conductor elements to a first node and a second node connection connecting the second ends of the conductor elements to a second node. The nodes may in turn be connected to an RF signal line for driving the conductor array or receiving signals from the conductor array. The first and second node connections may provide equal impedance paths between each of the ends and the respective nodes. This may be done by providing equal path links between each end and the respective node and, more particularly, by providing a set of separate equal length branches from a signal node, each branch branching again into a second set of separate equal length branches which ultimately connect to the ends of the conductor elements. The connection of the conductor elements of the arrays thus promotes equal current through each conductor element, simplifying construction of the resulting field and improving its homogeneity.
In a second preferred embodiment, the first and second node connections provide substantially non-overlapping straight line paths between the respective ends and the node. This may be realized by a substantially continuous isosceles triangular conductor having its node at the apex and the ends of the conductor elements distributed along the base of the isosceles triangular conductor. In this manner a lowest possible resistance connection between each of the conductor elements and the node is provided.
The coil of the invention may include an RF shield for a quadrature coil, the latter having a first conductor array and a second perpendicular conductor array. The RF shield provides a conductive surface interrupted by channels substantially aligned with the conductor elements of both the first and second conductor arrays. The channels of this RF shield prevent eddy current formation caused by excitation of gradient coil fields such as might interfere with the RF coil and/or reduce the power or affect the shape of the gradient coils.
The channels aligned with the conductor elements of the first conductor array may be on a first conductive sheet and the channels aligned with the conductor elements of the second conductor array may be on a second conductive sheet adjacent to the first conductive sheet. The channels may be bridged by capacitors sized to provide low admittance at the operational radio frequency. The RF shield is thus easily manufactured.
A quadrature coil set comprised of four crossing conductor arrays may be placed at the pole of the open frame MRI magnet, with a first and third conductor array being at opposite poles and having parallel conductor elements, and a second and fourth conductor array being at opposite poles and having parallel conductor elements perpendicular to the conductor elements of the first and third array. Interconnection leads may connect the first and third conductor arrays in series through their connection points and may connect the second and fourth conductor arrays and series through their connection points, thus promoting opposite current flows through the first and third conductor arrays and through the second and fourth conductor arrays.